强噪声下碳纤维增强树脂复合材料结构Lamb波层析损伤成像方法

Lamb波因其检测范围广、对缺陷敏感性高等特点在复合材料无损检测中广泛应用。但强噪声环境给有效信号的提取带来难度,影响损伤位置判定精度。针对该问题,提出了一种在强噪声背景下基于计盒维数和Lamb波层析成像技术的损伤定位成像方法。首先通过仿真分析了Lamb波在碳纤维增强树脂(CFRP)复合材料板损伤前后传播的特性。在选定的复合材料板上均匀布置圆形传感器阵列,以粘结质量块改变结构局部刚度的形式模拟真实损伤;其次每个传感器依次作为激励器产生Lamb波,其他传感器采集有无损伤下的响应信号,采用小波变换进行信号去噪。将去噪后的信号添加不同等级的白噪声实现噪声干扰;最后采用计盒维数计算有无损伤的信号差异确定损伤因子,并通过概率成像算法实现损伤的定位成像。实验结果表明,在强噪声环境中单损伤与多损伤成像定位最大和平均误差分别为11.18 mm和6.88 mm,该方法无需信号降噪技术,且避免了多损伤时复杂反射信号的提取过程,在强噪声下复合材料损伤定位识别方面具有较大的潜力。     

板状结构中基于Lamb波单模态的缺陷成像试验研究

Lamb波技术非常适于板类结构的各种缺陷检测。为了得到模态较为单一的Lamb波,使得接收信号更具有解释性,缺陷更容易识别,该文中在板上下表面的相同位置分别布置压电传感器阵列,采用同一位置上下表面的双压电片同相或反相激励,激励出单一S₀或A₀模态。运用这种单模态的Lamb波激励方式对板中缺陷进行检测,结合椭圆成像算法与数据融合方法对压电传感器阵列接收到的多组信号进行缺陷成像。该方法有效的实现了板中缺陷的二维成像定位,检测出人工缺陷,并提高了缺陷的可识别能力和定位精度。     

基于互相关函数幅值向量和小波变换的复合材料结构损伤检测

提出了一种基于互相关函数幅值向量(Cross correlation function amplitude vector,corV)和连续小波变换(continuous wavelet transform,CWT)联合检测损伤的方法.选取双正交小波函数,首先计算随机激励下完好结构和损伤结构的互相关函数幅值向量,再分别对其进行连续小波变换,根据损伤前后小波系数的变化来检测结构的损伤,并以小波系数差的模最大值作为损伤指标来进行损伤定位.该方法无需对结构建模,仅需获取结构损伤前后的时域响应信号.通过对带有不同位置分层损伤的复合材料层合梁和层合板损伤检测的数值仿真,验证了用该方法检测复合材料结构分层损伤的有效性.     

基于阵列传感器的复合材料结构损伤MUSIC成像方法

提出了一种基于传感器线阵的多重信号分类(Multiple Signal Classification,MUSIC)损伤成像方法用于航空复合材料的损伤监测。该方法采用MUSIC阵列信号处理方法,通过对传感器阵列信号进行协方差特征值分解,在结构上进行方向扫描并构建监测区域的空间谱,从而实现对结构损伤的成像,具有一维传感器阵列易于布置的优点。所提出的方法在变厚度航空复合材料油箱结构上进行了验证,结果表明,该方法能够准确实现航空复合材料结构上的损伤成像,定位误差小于2cm。     

基于改进空间滤波器的复合材料结构损伤成像方法

为降低Lamb波在复合材料结构中传播时存在的频散现象和各方向传播特性的不一致性给损伤监测带来的困难, 提出一种利用Hilbert变换改进的空间滤波器对复合材料结构损伤进行成像的方法。分析了基于空间滤波器的信号合成成像原理, 在此基础上利用Hilbert变换构造传感器时域响应信号的解析信号参与到信号合成成像过程中。通过对仿真声源的成像实验, 验证了该方法的可行性, 通过对碳纤维复合材料板结构上激励源和损伤的成像实验, 验证了该方法的功能。研究表明该方法能够识别损伤相对于压电传感器阵列的角度位置和损伤散射信号的到达时刻。      

基于MUSIC及波束成形算法的非线性Lamb波微裂纹成像与评估EI北大核心CSCD

针对板状结构的微裂纹成像和评估问题,使用有限元分析方法在三维铝板中建立了埋藏型微裂纹,并采用对称激励的方式,在板中激励出S模态的Lamb波。利用多信号输入分类(multiple signal classification, MUSIC)算法,对传感器阵列捕捉到的信号进行处理,测算板中位于远场的微裂纹方向角。而后通过设置多组传感器阵列进行交叉定位,从而实现对板状结构中可能存在的微裂纹的大范围、高精度定位成像。以此为基础,对阵列信号进行波束成形算法处理,实现对信号的空间滤波,以减少其他方向的杂波对阵列信号的干扰;通过对处理后的阵列信号进行分析,计算其非线性参数,实现对裂纹损伤几何信息的评估。研究结果表明,该研究所构建的微裂纹定位成像算法在较高的环境噪声干扰下仍具备较好的损伤成像能力,且计算出的非线性参数与微裂纹厚度和宽度密切相关,可以有效评估微裂纹程度。     

闭合型裂纹时间反转损伤识别

由于超声波能穿透闭合型裂纹界面,采用传统的线性超声技术难以检测和定位闭合型裂纹损伤,基于非线性时间反转理论研究了闭合型裂纹损伤检测方法。通过金属铝板上黏贴的压电激励/传感阵列,进行了Lamb波检测螺栓模拟的闭合型裂纹实验。根据Lamb波时间反转原理,得到激励重构信号。通过计算激励信号和重构信号间的相关系数定义了损伤指数,开发了能定位闭合型裂纹的成像算法。实验结果表明非线性超声时间反转方法能够有效地识别非线性的闭合型裂纹。     

基于小波包能量比变化率偏差的复合材料层板空气耦合超声概率损伤成像

传统椭圆概率损伤成像使用信号幅值差或能量差作为特征参量,对损伤识别不够敏感且抗噪性能较差。为提高损伤识别的敏感程度,提出采用小波包能量比变化率偏差(Energy relative variation deviation,ERVD)作为损伤因子。选取合适的探头入射倾斜角度,使用空气耦合超声探头在复合材料层板中激励单一的Lamb波模态,对采集的扫查信号进行小波包分解,根据结构损伤前后的信号特征变化选取特征频带,计算损伤指数进行椭圆概率损伤成像,并模拟不同噪声环境对比不同损伤因子的成像效果差异。实验结果表明,选取小波包能量比变化率偏差作为损伤因子,具有较强的损伤识别敏感性和抗噪性能。使用此损伤因子进行空气耦合超声概率损伤成像,可提高复合材料损伤的定位和成像效果。      

基于矩形脉冲涡流传感器的表面缺陷定量评估及成像检测技术研究

本文介绍利用矩形脉冲涡流传感器的自差分特性来提高缺陷检测能力,并最终实现对表面缺陷的成像检测.采用ANSYS软件建立了矩形脉冲涡流传感器的仿真模型,对铝板表面涡流分布进行了仿真分析,结果表明:矩形传感器能够在铝板表面激励出均匀的感应涡流,当有缺陷存在时,提取Z向和Y向感应信号的幅值扫描曲线可以实现对表面缺陷长度和深度的定量.并对仿真结果进行了实验验证,得到了表面缺陷的成像结果.研究结果表明,该矩形脉冲涡流传感器可以很好的实现对表面缺陷的定量评估及成像检测.     

基于涡流成像的碳纤维增强树脂基复合材料细观结构可视化

对碳纤维增强树脂基复合材料（CFRP）的细观结构成像方法进行了研究,利用涡流成像技术实现了CFRP层合板中纤维方向及纤维缺失、褶皱和空隙过大等缺陷的可视化。首先通过有限元仿真和电路理论分析了CFRP板中涡流的生成机制和分布特性,阐述了基于涡流法的CFRP细观结构成像机制。然后介绍了用于扫描成像的高频涡流检测（HF-ECT）实验系统并确定了涡流探头的形式及其参数。最后利用涡流成像技术分别对单层板、正交层合板和四方向斜交层合板进行了检测,绘制了涡流检测（ECT）信号的三维伪彩图并得到了清晰的纤维纹路分布。通过引入滤波去噪技术和二维快速傅里叶变换（2D-FFT）对图像进行进一步处理,提高了图像分辨率并完成了不同方向上纤维纹路的分离,从而实现对层合板每单向层中缺陷的精确定位。     

Structural Health Monitoring Using Lamb Wave Reflections and Total Focusing Method for Image Reconstruction

This investigation aimed to adapt the total focusing method (TFM) algorithm (originated from the synthetic aperture focusing technique in digital signal processing) to accommodate a circular array of piezoelectric sensors (PZT) and characterise defects using guided wave signals for the development of a structural health monitoring system. This research presents the initial results of a broader study focusing on the development of a structural health monitoring (SHM) guided wave system for advance carbon fibre reinforced plastic (CFRP) composite materials. The current material investigated was an isotropic (aluminium) square plate with 16 transducers operating successively as emitter or sensor in pitch and catch configuration enabling the collection of 240 signals per assessment. The Lamb wave signals collected were tuned on the symmetric fundamental mode with a wavelength of 17 mm, by setting the excitation frequency to 300 kHz. The initial condition for the imaging system, such as wave speed and transducer position, were determined with post processing of the baseline signals through a method involving the identification of the waves reflected from the free edge of the plate. The imaging algorithm was adapted to accommodate multiple transmitting transducers in random positions. A circular defect of 10 mm in diameter was drilled in the plate, which is similar to the delamination size introduced by a low velocity impact event in a composite plate. Images were obtained by applying the TFM to the baseline signals, Test 1 data (corresponding to the signals obtained after introduction of the defect) and to the data derived from the subtraction of the baseline to the Test 1 signals. The result shows that despite the damage diameter being 40 % smaller than the wavelength, the image (of the subtracted baseline data) demonstrated that the system can locate where the waves were reflected from the defect boundary. In other words, the contour of the damaged area was highlighted enabling its size and position to be determined.     

基于经验模态分解和相关系数对玻璃纤维增强聚合物复合材料板的损伤识别及扫查成像

针对外界环境噪声等因素造成损伤因子不敏感，导致复合材料损伤识别困难和成像误差大等问题，提出了一种基于经验模态分解（Empirical mode decomposition, EMD）和相关系数的损伤因子。用空气耦合探头采集损伤前后的Lamb波信号进行EMD分解获取多个本征模态分量（Intrinsic mode function, IMF）。根据相关系数获取与信号相关性最大的IMF分量，并定义其能量值的相对变化为损伤因子。在模拟噪声环境前后，分别对玻璃纤维增强聚合物复合材料（GFRP）板中的分层缺陷进行识别和扫查成像，验证了该损伤因子的有效性。结果表明:信号经过EMD分解后，与其相关性最大的IMF分量对损伤最敏感，能够定义为识别损伤的损伤因子。将该损伤因子结合概率成像方法进行空耦Lamb波扫查，不仅能够有效对复合材料中的缺陷进行成像，而且在模拟强噪声环境中具有良好的抗噪性。      

频率-波数域的薄铝板缺陷检测研究

文章将频率-波数域成像算法应用于薄铝板内圆形穿孔缺陷的检测。采用PZFlex软件建立多阵元线性阵列仿真模型,在薄铝板内激发出Lamb波,模拟超声相控阵全矩阵捕获功能。实验中使用M2M公司的超声相控阵仪器对带有通孔缺陷的薄铝板进行全矩阵捕获。基于全矩阵数据完成自发自收模式下的频率-波数法损伤重建,对全矩阵数据进行三维傅里叶变换,利用所有阵列的全部信息完成损伤重建。实验结果表明:频率-波数域成像法能够精确地表征出圆孔损伤的大小和形状,对单缺陷及不同大小、距离的双缺陷均具有良好的检测效果。相对于全聚焦成像法,频率-波数域成像法具有更高的分辨率。     

Effects of local stiffness changes and delamination on Lamb wave transmission using surface-mounted piezoelectric transducers

The paper presents a set of numerical results on the use of surface mounted piezoelectric transducers to analyse the effects of impact damage and delamination of plate-like structures on the Lamb wave mode. The effects of the size, properties and orientation of the damage upon a propagating Lamb wave is qualitatively determined. In this paper, impact damage was simulated by a local change in the stiffness of the material in the structure and a delamination. The effects on the transmission of the incident Lamb wave when it propagates through a region of change in density are analysed. This paper will also demonstrate how the properties of a propagating Lamb wave can be affected by the existence of a delamination in a plate.     

Damage assessment in layered composites using spectral analysis and Lamb wave

Piezo-ceramic transducers of the surface mounted type are commonly used for structural health monitoring (SHM) techniques. But, there is a disadvantage to use piezo-ceramic transducers of the surface mounted type in Lamb wave application. Due to the symmetric and antisymmetric Lamb wave modes generated by the surface mounted piezo-ceramic transducers simultaneously, the received signals are very complex and it is difficult to extract damage information from the signals.

In this paper, the practical method for SHM was proposed using piezo-ceramic transducers of the surface mounted type and Lamb wave. In order to overcome the difficulties in the signal processing of the simultaneous modes, the symmetric and antisymmetric modes were separated by using the two sensors bonded on the opposite surfaces at the same point. Also, spectral analyses of the separated symmetric and antisymmetric Lamb waves showed that each mode propagated with different frequency characteristics in the exciting frequency range.

By making use of these findings, the changes of power spectrum density in characteristic frequency band of symmetric and antisymmetric modes are proportional to the delamination size in quasi-isotropic Gr/Ep laminates. Therefore, this paper presents the damage assessment technique to extract damage information from the complicated PZT signals that could not be interpreted in time domain.     

Temperature Independent Damage Detection in Plates Using Redundant Signal Measurements

This paper describes a method to detect notch like damages in plates using piezoelectric transducers. The method does not use prior baseline data for damage detection. A single pair of piezoelectric wafer transducers made of Lead Zirconate Titanate (PZT) is attached back to back on the opposite sides of a plate and are used for simultaneous actuation and sensing. A notch, which is a sudden change in thickness of the plate, leads to mode conversion of Lamb waves. The mode converted wave component in the measured signal is then separated from the other Lamb wave mode components using polarization characteristics of the piezoelectric wafer transducers. The damage index is a function of the amplitude of this mode converted component of the signal. In real world situations, the damage index will not be exactly zero due to inaccuracy in transducer collocation and non-uniformity in their bonding conditions. Therefore, a (non-zero) threshold for the damage index needs to be established to avoid false alarms. True to the spirit of baseline-free damage detection, this threshold is computed from the signals acquired only from the current state of the structure. This is achieved by using redundancy in signal measurements. Since the method detects damages without having to rely on baseline data, environmental variations like temperature change do not affect its performance. Results from numerical simulations as well as experiments on aluminum specimens are provided to demonstrate the effectiveness of the method described above.     

密集型矩形阵列参数对激光Lamb波成像的影响分析

激光超声技术具有非接触、检测效率高等优点,在无损检测领域受到广泛关注;充分利用激光超声技术的高空间分辨率特性,结合密集型矩形阵列和激光Lamb波技术进行板中缺陷检测。采用连续小波变换对频带宽、时域分辨率低的激光Lamb波信号进行提取,得到特定频率下具有高时域分辨率的窄带信号;利用线性映射补偿技术消除所提取窄带信号中的频散,消除频散的信号用于缺陷成像;最后,结合幅值成像技术和符号相干因子成像技术对频散补偿后的信号进行处理,实现铝板中缺陷的成像和定位。在此基础上,进一步对不同的阵元数量和阵元间距对密集型矩形阵列指向性和缺陷成像质量的影响进行分析。当阵元数量为16,阵元间距为一个Lamb波波长时,主瓣宽度较窄且没有栅瓣出现,缺陷成像质量得到有效提高。     

分谱处理算法在基于超声导波损伤识别中的应用

利用分谱处理（SSP）算法,通过比较基准波信号和检测波信号的瞬时相位变化度（IPVD）来评估损伤散射的基础阶对称（S0）模式的飞行时间（ToF）,最终实现基于超声导波的损伤识别。首先在理想的工作环境中（无噪声）,对无缺陷的铝板进行检测,并把所采集的波信号作为基准信号。然后在不同的工作环境中（无噪声和有噪声）检测带有切缝缺陷的铝板,并把所采集的波信号作为检测信号。实验结果表明,当检测波信号的信噪比较低时,噪声能量严重干扰了检测波信号的能量分布。然而,SSP算法所提取的检测波信号的IPVD几乎不受噪声的干扰,因此利用SSP算法能够有效地抵抗强噪声的干扰,精确地评估损伤散射的S0模式的ToF,并进一步结合三角定位算法成功地定位出铝板中的切缝缺陷。     

Guided Wave Damage Characterization via Minimum Variance Imaging with a Distributed Array of Ultrasonic Sensors

Guided wave imaging with a distributed array of inexpensive transducers offers a fast and cost-efficient means for damage detection and localization in plate-like structures such as aircraft and spacecraft skins. As such, this technology is a natural choice for inclusion in condition-based maintenance and integrated structural health management programs. One of the implementation challenges results from the complex interaction of propagating ultrasonic waves with both the interrogation structure and potential defects or damage. For example, a guided ultrasonic wave interacts with a surface or sub-surface defect differently depending on the angle of incidence, defect size and orientation, excitation frequency, and guided wave mode. However, this complex interaction also provides a mechanism for guided wave imaging algorithms to perform damage characterization in addition to damage detection and localization. Damage characterization provides a mechanism to help discriminate actual damage (e.g. fatigue cracks) from benign changes, and can be used with crack propagation models to estimate remaining life. This work proposes the use of minimum variance imaging to perform damage detection, localization, and characterization. Scattering assumptions used to perform damage characterization are obtained through both analytical and finite element models. Experimental data from an in situ distributed array are used to demonstrate feasibility of this approach using a through-hole and two through-thickness notches of different orientations to simulate damage in an aluminum plate.